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Control methods

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Dandelion (Taraxacum officinale)

Photo: Jardin botanique de Montréal (Réjean Martel)

Looking for a way to control well-established weeds? Here are the main control methods

Manual pulling

The best way to eliminate most undesirable plants is to pull them out manually, eradicating as much of the roots as possible. This is easier to do when the soil is moist and friable. In all cases, it is essential that undesirable plants be pulled out before they produce flowers or seeds.

Dandelion diggers are effective in removing weeds that form rosettes of leaves (dandelions and plantains, for example) on the lawn. The ones with metal tongs are very effective, but they leave holes that need to be filled with soil and reseeded, otherwise weeds will take over again.

Manual pulling is less effective in plants with long rhizomes, as the small fragments remaining in the soil will generate new growths.

False seedbed

This technique, used before seeding or planting, consists of loosening the soil and letting weeds sprout so as to then eliminate them. For best results, prepare the soil a few weeks ahead of time, when it is hot and humid out. You can even water the soil and cover it with a clear plastic to stimulate the germination of weeds’ seeds. Start seeding and planting as soon as the weeds have been eliminated.

A highly infested plot may require several weeding sessions before seeding or planting, as the false seedbed will not necessarily eliminate all weeds on the first try. For this method to be effective, all undesirable plants must be pulled out before proceeding with the false seedbed, the goal being to eliminate the “seed bank” present in the soil.

Physical barrier

A rigid metal or plastic border may limit the development of undesirable plants that spread thanks to their underground stems (rhizomes). To be effective, this barrier must penetrate the first 15 to 20 centimetres of the soil. There are some limitations with this technique, however. They are not as effective with weeds whose rhizomes are deep within the soil, such as common horsetail and Japanese knotweed.

Hoeing

When done regularly, hoeing helps eliminate young weed plants. Hoeing is ideally done on a sunny day, when the ground’s surface is dry to prevent weeds from taking root again. Avoid hoeing too often, as this can dry out the soil. Hoeing also disturbs the soil and promotes the germination of dormant seeds. Be careful not to damage the roots of food and ornamental plants while hoeing.

Mulch renders hoeing useless. Mulching is by far better than hoeing.

Thermal treatment

Weeds between paving stones or bordering the sidewalk can be burned using a propane gas device with torch. There is no need to scorch weeds. Just pass the flame over the plant until it discolours, without browning it completely. Unfortunately, this tool uses a non-renewable resource and produces CO2 (a greenhouse gas). This is not the case with boiling water, which also gives good results.

Thermal treatments are more effective with annuals. A few treatments are required to eliminate perennials.

Low-impact herbicides

As a last resort, you can use herbicides whose active ingredients are considered to have low toxicity for human health and the environment.